SiC has excellent physical property values including the forbidden-band width of 3 eV or greater and the extremely high breakdown field intensity of 2.5 MV/cm or greater. Thus, it is gaining attention as a semiconductor material that can be used for realizing an ultralow-loss power transistor, a high-output, high-frequency transistor, or a field effect transistor having an extremely small gate length, all of which cannot be realized theoretically with the existing semiconductors, such as Si and GaAs.
However, when a metal-silicon oxide film-semiconductor field effect transistor (MOSFET), which is the basic structure of field effect transistor devices having an insulated gate, is prepared using SiC, the effective channel electron mobility at the SiO2/SiC interface becomes much smaller than the electron mobility of bulk SiC, resulting in an increase in channel resistance. As a result, the on-characteristics of the transistor deteriorate, making the realization of high-performance devices difficult.
The cause for the extremely small effective channel electron mobility is believed to exist in the SiO2/SiC interface. Various measures have so far been taken with regard to the SiO2/SiC formation processes and the like, such as analysis of oxidation temperature and oxidative atmosphere of SiC, addition of nitrogen to a silicon oxide film, and changes in the orientation of SiC crystal that is oxidized.
Meanwhile, there is another attempt to use an insulating film other than SiO2 as the gate insulating film. Contemplated substances include amorphous substances similar to the oxide film and single-crystal substances similar to SiC. Of such substances, attention is being focused on AlN because it has a hexagonal crystal structure without inversion symmetry, as in SiC, and because the lattice constants of AlN and SiC are relatively close. A report has already been made regarding a prototype of a metal/AlN/SiC-based metal-insulator-semiconductor field effect transistor (MISFET) (see Non-Patent Document 1, for example).
However, it has been very difficult to grow a high-quality AlN crystal on the surface of SiC. Because the only AlN layer that has been available was of low quality, a large leakage current flows in the AlN layer. As a result, the AlN/SiC-based MISFETs that have been made have very poor gate insulating property in particular, and no devices have been realized that are suitable for practical application.
Lately, the present inventors have found a method for growing a high-quality AlN crystal on the SiC surface and succeeded in the growth of an AlN layer crystal that has a very good insulting property. Nevertheless, even if the AlN is of high quality, it has been theoretically and experimentally verified that, because the conduction-band discontinuous quantity between AlN and SiC is approximately 2.0 eV, which is not much larger than the discontinuous quantity of 2.74 eV between SiO2 and SiC, if an electric field of close to 3 MV/cm is applied to the AlN layer, electrons tunnel through the AlN layer due to quantum-mechanical tunneling effect, whereby gate insulating property is lost (see Non-patent Document 2, for example).
Non-patent Document 1: C.-M. Zetterling, M. Ostling, H. Yano, T. Kimoto, H. Matsunami, K. Linticum and R. F. Davis, “SiC MISFETs with MBE-grown AlN Gate Dielectric”, Material Science Forum Vols. 338-342(2000) pp. 1315-1318.
Non-patent Document 2: Norio Onojima, Jun Suda, and Hiroyuki Matsunami, “Molecular-beam epitaxial growth of insulating AlN on surface-controlled 6H—SiC substrate by HCl gas etching”, Applied Physics Letters, Vol. 80, No. 1, (2002) p. 76-78.